The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services beyond the early voice-oriented services. In line with this tendency, the standardization organizations such as 3rd Generation Partnership Project (3GPP), 3GPP2, and Institute of Electrical and Electronics Engineers (IEEE) are standardizing 3G evolved mobile communication standards based on multicarrier multiple access scheme. The 3GPP Long Term Evolution (LTE), 3GPP2 Ultra Mobile Broadband (UMB), and IEEE 802.16m are the mobile communication standards that have been developed to support high speed high quality wireless packet data communication services based on the multicarrier multiple access scheme.
Existing 3G evolved mobile communication standards such as LTE, UMB, and IEEE 802.16m based on the multicarrier multiple access scheme are characterized by various techniques including Multiple Input Multiple Output (MIMO), beamforming, Adaptive Modulation and Coding (AMC), channel sensitive scheduling, etc. for improving transmission efficiency. Such techniques are capable of concentrating transmission power with multiple antennas or adjusting transmission data amount depending on the channel quality and transmitting data to the user with good channel quality selectively, resulting in improvement of transmission efficiency and increase of system throughput.
Because most of these techniques operate based on the channel state information between an evolved Node B (eNB) (or Base Station (BS)) and a User Equipment (UE) (or Mobile Station (MS)), the eNB or UE has to measure the channel state between the eNB and UE based on Channel State Indication Reference Signal (CSI-RS). The eNB is a transmitter in downlink and a receiver in uplink and capable of managing a plurality cells for communication. A mobile communication system is made up of a plurality of eNBs distributed geographically, and each eNB manages a plurality of cells to provide the UEs with communication service.
Existing 3G and 4G mobile communication systems represented by LTE/LTE-A adopt MIMO technique using a plurality transmission/receive antennas to increase data rate and system throughput. Using a MIMO scheme, it is possible to transmit a plurality of information streams separated spatially. This technique of transmitting the plural information streams is referred to as spatial multiplexing. Typically, the number of information streams to be spatially multiplexed is determined depending on the numbers of antennas of the transmitter and receiver. The number of information streams that can be spatially multiplexed is referred to as rank of the corresponding transmission. The LTE/LTE-A Release 11 supports 8×8 MIMO spatial multiplexing and up to rank 8.
The Full Dimension MIMO (FD-MIMO) system to which the method proposed in the present disclosure is applied is capable of using 32 or more transmit antennas with the evolvement of the legacy LTE/LTE-A MIMO scheme supporting up to 8 antennas.
The FD-MIMO system is the wireless communication system capable of transmitting data using a few dozen or more of transmit antennas.
FIG. 1 illustrates an example FD-MIMO system.
Referring to FIG. 1, the base station transmitter 100 transmits radio signals 120 and 130 through a few dozen or more transmit antennas. The transmit antennas 110 are arranged at minimum distance among each other. The minimum distance may be half of the wavelength (?/2). Typically, in the case that the transmit antennas are arranged at the distance of half of the wavelength of the radio signal, the signals transmitted by the respective transmit antennas are influenced by radio channel with low correlation. Assuming the radio signal band of 2 GH, this distance is 7.5 cm and shortened as the band becomes higher than 2 GHz.
In FIG. 1, a few dozen or more transmit antennas 110 arranged at the base station are used to transmit signals to one or more terminals as denoted by reference number 120 and 130. In order to transmit signals to plural terminals simultaneously, an appropriated precoding is applied. At this time, one terminal may receive plural information streams. Typically, a number of information streams which a terminal can receive is determined depending on the number of receive antenna of the terminal, channel state, and reception capability of the terminal.
In order to implement the FD-MIMO system efficiently, the terminal has to measure the channel condition and interference size accurately and transmit the channel state information to the base station efficiently. If the channel state information is received, the base station determines the terminals for downlink transmission, downlink data rate, and precoding to be applied. In the case of FD-MIMO system using large number of transmit antennas, if the channel state information transmission method of the legacy LTE/LTE-A system is applied without modification, the control information amount to be transmitted in uplink increases significantly, resulting in uplink overhead.
The mobile communication system is restricted in resource such as time, frequency, and transmission power. Accordingly, if the resource allocated for reference signal increases, the resource amount to be allocated for data traffic channel transmission decreases, resulting in reduction of absolute data transmission amount. In this case, although the channel estimation and measurement performance are improved, the data transmission amount decreases, resulting in reduction of entire system throughput.
Thus, there is a need of allocating the resources for reference signal and traffic channel transmissions efficiently in order to maximize the entire system throughput.
FIG. 2 illustrates an example time-frequency grid a single Resource Block (RB) of a downlink subframe as a smallest scheduling unit in the LTE/LTE-A system.
As shown in FIG. 2, the radio resource is of one subframe in the time domain and one RB in the frequency domain. The radio resource consists of 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, i.e. 168 unique frequency-time positions. In LTE/LTE-A, each frequency-time position is referred to as Resource Element (RE).
The radio resource structured as shown in FIG. 2 can be used for transmitting plural different types of signals as follows.
1. CRS (Cell-specific Reference Signal): reference signal transmitted to all the UEs within a cell
2. DMRS (Demodulation Reference Signal): reference signal transmitted to a specific UE
3. PDSCH (Physical Downlink Shared Channel): data channel transmitted in downlink which the eNB use to transmit data to the UE and mapped to REs not used for reference signal transmission in data region of FIG. 2
4. CSI-RS (Channel state information Reference Signal): reference signal transmitted to the UEs within a cell and used for channel state measurement. Multiple CSI-RSs can be transmitted within a cell.
5. Other control channels (PHICH, PCFICH, PDCCH): channels for providing control channel necessary for the UE to receive PDCCH and transmitting ACK/NACK of HARQ operation for uplink data transmission.
In addition to the above signals, zero power CSI-RS can be configured in order for the UEs within the corresponding cells to receive the CSI-RSs transmitted by different eNBs in the LTE-A system. The zero power CSI-RS (muting) can be mapped to the positions designated for CSI-RS, and the UE receives the traffic signal skipping the corresponding radio resource in general. In the LTE-A system, the zero power CSI-RS is referred to as muting. The zero power CSI-RS (muting) by nature is mapped to the CSI-RS position without transmission power allocation.
In FIG. 2, the CSI-RS can be transmitted at some of the positions marked by A, B, C, D, E, F, G, H, I, and J according to the number of number of antennas transmitting CSI-RS. Also, the zero power CSI-RS (muting) can be mapped to some of the positions A, B, C, D, E, F, G, H, I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs according to the number of the antenna ports for transmission. For two antenna ports, half of a specific pattern is used for CSI-RS transmission; for four antenna ports, entire of the specific pattern is used for CSI-RS transmission; and for eight antenna ports, two patterns are used for CSI-RS transmission. Meanwhile, muting is always performed by pattern. That is, although the muting may be applied to plural patterns, if the muting positions mismatch CSI-RS positions, muting cannot be applied to one pattern partially.
In the case of transmitting CSI-RSs of two antenna ports, the CSI-RSs are mapped to two consecutive REs in the time domain and distinguished from each other using orthogonal codes. In the case of transmitting CSI-RSs of four antenna ports, the CSI-RSs are mapped in the same way of mapping the two more CSI-RSs to two more consecutive REs. This is applied to the case of transmitting CSI-RSs of eight antenna ports.
In a cellular system, the reference signal has to be transmitted for downlink channel state measurement. In the case of the 3GPP LTE-A system, the UE measures the channel state with the eNB using the CSI-RS transmitted by the eNB. The channel state is measured in consideration of a few factors including downlink interference. The downlink interference includes the interference caused by the antennas of neighbor eNBs and thermal noise that are important in determining the downlink channel condition. For example, in the case that the eNB with one transmit antenna transmits the reference signal to the UE with one receive antenna, the UE has to determine energy per symbol that can be received in downlink and interference amount that may be received for the duration of receiving the corresponding symbol to calculate Es/Io from the received reference signal. The calculated Es/Io is reported to the eNB such that the eNB determines the downlink data rate for the UE.
In the LTE-A system, the UE feeds back the information on the downlink channel state for use in downlink scheduling of the eNB. That is, the UE measures the reference signal transmitted by the eNB in downlink and feeds back the information estimated from the reference signal to the eNB in the format defined in LTE/LTE-A standard. In LTE/LTE-A, the UE feedback information includes the following three indicators:
1. Rank Indicator (RI): number of spatial layers that can be supported by the current channel experienced at the UE.
2. Precoding Matrix Indicator (PMI): precoding matrix recommended by the current channel experienced at the UE.
3. Channel Quality Indicator (CQI): maximum possible data rate that the UE can receive signal in the current channel state. CQI may be replaced with the SINR, maximum error correction code rate and modulation scheme, or per-frequency data efficiency that can be used in similar way to the maximum data rate.
The RI, PMI, and CQI are associated among each other in meaning. For example, the precoding matrix supported in LTE/LTE-A is configured differently per rank. Accordingly, the PMI value ‘X’ is interpreted differently for the cases of RI set to 1 and RI set to 2. Also, when determining CQI, the UE assumes that the PMI and RI which the UE has reported are applied by the eNB. That is, if the UE reports RI_X, PMI_Y, and CQI_Z; this means that the UE is capable of receiving the signal at the data rate corresponding to CQI_Z when the rank RI_X and the precoding matrix PMI_Y are applied. In this way, the UE calculates CQI with which the optimal performance is achieved in real transmission under the assumption of the transmission mode to be selected by the eNB.
In LTE/LTE-A, the UE is configured with one of the following four feedback or reporting modes depending on the information to be included therein:
1. Mode 1-0: RI, wideband CQI (wCQI)
2. Mode 1-1: RI, wCQI, wideband PMI (wPMI)
3. Mode 2-0: RI, wCQI, subband CQI (sCQI)
4. Mode 2-1: RI, wCQI, wPMI, sCQI, sPMI
The feedback timing in the respective feedback mode is determined based on ICQI/PMI transmitted through high layer signaling and Npd, NOFFSET,CQI, MRI, NOFFSET,RI corresponding to IRI. In Mode 1-0, the wCQI transmission period is Npd, and the feedback timing is determined based on the subframe offset value of NOFFSET,CQI. The RI transmission period is Npd·MRI, and RI transmission period offset is NOFFSET,CQI+NOFFSET,RI.
FIG. 3 illustrates example feedback timings of RI and wCQI in the case of Npd=2, MRI=2, NOFFSET,CQI=1, and NOFFSET,RI=−1. Here, the each timing is indicated by subframe index.
Here, the feedback mode 1-1 has the same timings as the feedback mode 1-0 with the exception that PMI is transmitted at the wCQI transmission timing together.
In the feedback mode 2-0, the sCQI feedback period is Npd with offset NOFFSET,CQI. The wCQI feedback period is H·Npd with offset NOFFSET,CQI equal to the sCQI offset. Here, H=J·K+1 where K is transmitted through higher layer signal and J is determined according to the system bandwidth.
For example, J is determined as 3 in the 10 MHz system. This means that wCQI is transmitted at every H sCQI transmissions in replacement of sCQI. The RI period MRI·H·Npd with offset NOFFSET,CQI+NOFFSET,RI 
FIG. 4 illustrates example feedback timings of RI, sCQI, and wCQI in the case of Npd=2, MRI=2, J=3 (10 MHz), K=1, NOFFSET,CQI=1, and NOFFSET,RI=−1.
The feedback mode 2-1 is identical with the feedback mode 2-0 in feedback timings with the exception that PMI is transmitted at the wCQI transmission timings together.
Unlike the feedback timings for the case of 4 CSI-RS antenna ports as described above, two PMIs have to be transmitted for 8 CSI-RS antenna ports. For 8 CSI-RS antenna ports, the feedback mode 1-1 is divided into two sub-modes. In the first sub-mode, the first PMI is transmitted along with RI and the second PMI along with wCQI. Here, the wCQI and second PMI feedback period and offset are defined as Npd and NOFFSET,CQI, and the RI and first PMI feedback period and offset are defined as MRI·Npd and NOFFSET,CQI+NOFFSET,RI, respectively. If the precoding matrix indicated by the first PMI is W1 and the precoding matrix indicated by the second PMI is W2, the UE and the eNB share the information on the UE-preferred precoding matrix of W1W2.
For the 8 CSI-RS antenna ports, the feedback mode 2-1 adopts new information of Precoding Type Indicator (PTI) which is transmitted along with RI at period of MRI·H·Npd with the offset of NOFFSET,CQI+NOFFSET,RI. For PTI=0, the first and second PMIs and wCQI are transmitted, particularly the wCQI and second PMI at the same timing at a period Npd with an offset of NOFFSET,CQI. Meanwhile, the first PMI is transmitted at a period of H′·Npd with an offset of NOFFSET,CQI. Here, H′ is transmitted through higher layer signaling. For PTI=1, the PTI and RI are transmitted at the same timing, the wCQI and second PMI are transmitted at the same timing, and sCQI is transmitted additionally. In this case, the first PMI is not transmitted. The PTI and RI are transmitted at same period with the same offset as the case of PTI=0, and sCQI is transmitted at a period of Npd with an offset of NOFFSET,CQI. Also, the wCQI and second PMI are transmitted at a period of H·Npd with an offset of NOFFSET,CQI, and H is set to the same value as the case of 4 CSI-RS antenna ports.
FIGS. 5 and 6 illustrate example feedback timings for PTI=0 and PTI=1 with Npd=2, MRI=2, J=3 (10 MHz), K=1, H′=3, NOFFSET,CQI=1, and NOFFSET,RI=−1, respectively.
Typically, in the FD-MIMO using a plurality of transmit antennas, the number of CSI-RSs has to increases in proportion to the number of transmit antennas. In an exemplary case of LTE/LTE-A using 8 transmit antennas, the eNB has to transmit CSI-RSs of 8 ports to the UE for downlink channel state measurement. At this time, in order to transmit 8-port CSI-RSs, 8 REs has to be allocated for CSI-RS transmission in one RB as marked by A and B in FIG. 2. In the case of applying CSI-RS transmission scheme of LTE/LTE-A to FD-MIMO, the CSI-RS transmission resource increases in proportion to the number of transmit antenna. That is, the eNB having 128 transmit antennas has to transmit CSI-RS on 128 REs in one RB. Such a CSI-RS transmission scheme consumes excessive radio resources and thus causes shortage of resource for data transmission.